This invention relates to devices of the type that control the flow of conditioned primary air through a duct into a controlled space in response to a sensed condition, such as temperature, humidity, and the like. The flow of primary air through the box induces a flow of unconditioned air (usually return air) from a plenum into the box, where it becomes mixed with the conditioned primary air before entering the controlled space. The flow of secondary air from the plenum into the mixing box is normally controlled by a secondary air damper, so as to maintain a substantially constant flow of mixed air into the controlled space.
One of the problems encountered in central air conditioning systems incorporating such mixing boxes is the possible rapid spread of fire and smoke through the plenum and into the controlled space as a result of the induced secondary air flow. Separate fire dampers have been employed to prevent flow of secondary air into the mixing box when excessive temperature is detected. Such dampers ordinarily comprise a damper blade biased toward closed position and held open by a fusible link, which melts when heated to its melting point, thus permitting the damper blade to close. After the fusible link has melted, it must be replaced before the damper can be reset to open position. The resumption of normal air distribution is thus dependent upon the availability and installation of a new link. The necessity for carrying a stock of fusible links is undesirable. The time required of someone to replace the link could more profitably be spent on other matters. Testing of the operation of the fire damper is impossible, since the fusible link is destroyed by application of sufficient heat to operate the fire damper. Another fire damper is held in open position by a latch responsive to a thermostatic bimetal. Upon detection of an excessive temperature, the latch is withdrawn, releasing the normally-closed damper and thereby permitting it to close. This type necessitates the resetting of the latch after each operation. The installation of such separate fire dampers also increases the installation cost.